 Thank you very much, Franco, for the very nice introduction. It's a pleasure to be here. It's a pleasure to see you again, and so many friends I'm seeing in the chat. I would like to thank also Chiara Righi. Let me take the opportunity, in fact, to thank also Roberto Zanin and Ulysses Barris de Almeida for the kind invitation. And my collaborators, Covino and Stefano Covino and Pietro Schipani in this particular enterprise for their collaboration and support. So I will start essentially from the basic light polarization. We know we learn very early in our curriculum, in our scientific training that light is composed by an electric and magnetic field component that oscillates perpendicularly to the direction of propagation. If some polarizing element, obstacle, scattering surface, whatever is interposed in the path, this element may select one of these oscillation planes and polarize the light on that plane. You can have this polarization essentially, is therefore can be considered as a breaking of symmetry, produced in a radiative source, we will see later, especially by magnetic field, or by intrinsically, locally, or by something, some obstacle, something interposed, intervening between the observer, source and the observer. You see various examples here of electron scattering, dust scattering and absorption, or surface scattering, all having the same result, the same final outcome of creating a preferential direction for the polarization, for the propagation, plane of propagation of light. We know that astrophysical sources show polarization in fact, at many wavelengths from radio to gamma rays, in fact. We are uncovering with time, there are spectacular recent results of X-ray, very significant X-ray polarimetry that I will patch on, I will show something. And this is caused, these essentially this polarizational these essentially this polarizational signals, polarized signals are related to intrinsic geometry of the source, a preferential geometric direction, like a symmetric axis, scatter radiation as we were seeing before in the previous slide, and again, scattering of radiation by magnetic fields. So also magnetic fields can be considered a scattering element with their preferred direction. They force particles, leptons or even protons to generate and they create naturally a direction or plane of propagation for the light that will therefore appear as polarized. So here are some, we will focus on a particular class of targets of polarized sources during this talk, which are the transients of interest for the Cherenkov telescope array, of course. So transients that are emitting in gamma rays and particularly in very high energy gamma rays. However, so I will show just with this slide how ubiquitous and heterogeneous polarization can be and how much information we can derive from polarized light in addition, complementary and added to the total light of a variety of sources. You can see here, for example, on the top left you see a protoplanetary disk around a rather famous bright star, E.B. Aurigen, as seen by the sphere instrument at the VLT. And the inner, the central twists are thought that are seen, polarized light are thought to be related to a newly born planet. Around this star, below you can see a spectacular optical and infrared image, again, taken at the VLT. The infrared is with a simple mode with sphere. And the infrared is from adaptive optics, which shows a binary system, a butterfly shape of a nebula, a planetary nebula that whose shape is produced, is caused by the presence of a companion that is, and this is uncovered, can be seen because we have, because we have these observations in polarized light. The bottom right shows one of the most queer objects in the planetary environment. This is a very peculiar, very rare type of comet. It's a recent one, and it's been observed in polarized light, again with force. And its high level of polarization indicates that it's a rather pristine object, namely it has not interacted too much with any, with the ambient medium, with any star. And it's the second object ever coming close to our sun from the external outer solar system. The more closer to the interest of transients, of high energy relativistic sources, is polarization in supernovae, which is also a relatively young, relatively recent type of detection. Type 1A supernovae are not expected to be high energy sources, gamma ray sources, but they do exhibit these are thermonuclear explosions. So thermonuclear burning complete, thermonuclear burning of a white dwarf by accretion of, after accretion of material from a companion. Occasionally, they have in their photospheres, they exhibit many absorption lines. Occasionally, these areas, these absorption line areas are exhibit some low level of polarization, which indicates not that much asymmetry in the explosion, which will be more related to polarization in the continuum. Polarization in the line indicates that there is a very irregular, clumpy medium in the outer that the outer ejecta are interacting with. And so these observations or these polarization observations in the area, in the wavelength ranges of absorption lines, or 1A supernovae, bear a lot of information on the medium surrounding the explosion. So let's go more into our zone of comfort, which is the CTA targets, the CTA sky, the TV sky. This view graph shows a map, the TV map of sources as we know it today. It's a relatively low number of sources compared to other wavelength ranges. There are so far only 200, less than 300 TV sources detected, about 1 fourth and 1 quarter of which are not identified. Those that have been identified, you see them here color coded. In fact, here you see more or less everything. The different colors indicate different sources. The majority of the galactic ones are obviously concentrated on the plane. The rest, the extra galactic ones, are predominantly blazers. So active galactic, radio loud, active galactic nuclei with very powerful relativistic jets pointing close to the observer direction, as we will see in a moment. And a few of them, a handful, in fact, of these red extra galactic sources are gamma-ray bursts. TV detection of gamma-ray bursts is recent. There are, as I say, a handful. And I have only highlighted the most famous one, the GRB of 9 October last year, the brightest of all times, because it's so powerful and since it is located a relatively low rate shift for a gamma-ray burst, it was detected as a very extremely bright source at all wavelengths. And we will see something about this guy later on. So these TV sources, so particularly the extra galactic ones, are all characterized by a common property. Their spectra are dominated by non-thermal processes, namely synchrotron radiation and the higher energies, occasionally inverse photons scattering of relativistic particles of photons, which are either intrinsic photons, namely synchrotron photons themselves, synchrotron self-compton, or external photons in the medium, external photons, fields of photons that are in the area, like emission line photons, accretion disk photons, even external photons. In some cases, we have, we do see in blazers also this type of scattering. But the common prey to the common feature is non-thermal radiation, which we know can be intrinsically very highly polarized. As we know from our studies, the Rebeke-Leitmann radiative processes textbook tells us and demonstrates that synchrotron radiation can be as high as 75%. So it is very difficult, very hard to see to observe this high level of polarization. But we will see with blazers, we get very close to it. And so let's start from the most famous non-thermal source in the extragalactic sky, the most famous jet in the sky. The sky of the radiojet of M87, which has actually been detected at all wavelengths. M87, in fact, is also a TV source, which is not seen as structured in TV as the lower energies, but of course, makes it extremely interesting. So you see here the jet of the source observed at various wavelengths and various scales, starting from the HST observations in optical. The length of the optical jet is about 10 arc seconds. So HST is particularly suited to map the bright notes along this jet. Then there is a zoom in with ALMA. This is polarized light. This is radiopolarization with ALMA that shows an extremely interesting and complex distribution of linear polarization along the jet. Then again, the VLBA that zooms in on the core of the ALMA image. And finally, the event horizon telescope, the very famous event horizon telescope, the millimetric image of the area, of the close synchronized area of the object of the core. And you see this is a polarized image. So if you observe, if you look at this image, this final HST image, you will see there are very thin curves that represent the reconstructed polarization, polarized lines. And so the analysis of these lines, these lines vary with time. There is a time variable millimetric detection, the millimetric detection of polarization is time value. There are variations between these observations. They globally show a quite high level of polarization, of 15%, which is, however, not uniform. It's not overall. It's concentrated in the southwest area of the source. And the application of magneto-hydrodynamics codes with relativistic corrections, relativistic assumptions, based on the total intensity and polarized linear polarization maps could be used, could be applied to reconstruct some physical parameters very close to the black hole. So a magnetic field of a few, between few and 30 gauss, and an accretion rate of 10 to the minus 4 solar masses per year. These radio jets, as we know, are ubiquitous, are very common in the sky. They are detected as radio galaxies, not most of which are much farther than M87. And when we see them, we see the two jets quite clearly and superluminal motions, as we will see in a moment, they have a variety of spectacular manifestations in spatial manifestations in the radio wavelengths. Occasionally, these jets point to very small angles, like 5, 3 degrees, with respect to the line of view, to the observer angle. Therefore, we don't see them as radio galaxies, and we see them as blazers, as we call them, which are simply radio galaxies with huge aberrations, special relativity aberration effects, because this plasma that moves along these two is collimated and moves at relativistic speeds along the jet directions and form kiloparsec jets, compressed spatially on the sky. We see a projection of these jets on the sky, a very tiny projection. And most often, we just see them as radio-compact cores. We don't see the jet at all. But we see all of the relativistic effects, like time, time scales for shortening, blue shifts of the spectrum, and, of course, magnification of the light, of the luminosities, and strong polarization, especially linear polarization. As a matter of fact, as explained in this milestone, a historical review of Angel and Stockman, strong polarization is the defining characteristic of blazers. Blazers are those sources, radio sources, line-less, optically featureless, or with lines, blazers, with extremely strong radio cores, flat radio spectra, and very strong polarization. And we know a bit more of these sources these days. This is an old view graph that shows the second Fermilat catalog, so the fraction of the sources that are bright in MEVG energy. So it's not very up-to-date, because we are now at the fourth catalog. But the gist is the same. The content of a fourth catalog image would be the same, namely, the majority, the vast majority of these sources, of these LAT sources, are indeed blazers. And in fact, identification, painstaking identification work of unidentified LAT sources is systematically leading as expected to the result that also enlarged the vast majority, large fraction of the unidentified Fermilat sources are blazers themselves. So the sample of gamma ray blazers, or MEVGV blazers, is just increasing. They are the dominant source of gamma rays, or MEV, to TV rays in the extragalactic sky. So let's see a little bit about their polarization. These are just two examples of polarized light, polarization of two very famous blazers. 3C279 is one of the brightest and most intensely variable. It is a TV source despite the relatively high redshift, although many extragalactic TV sources have been recently detected at redshifts even larger than this one, 0.54. And it shows large optical variability you see here on the top left. And the linear polarization percentage in the bottom together with the radio polarization. And you can appreciate here the good correlation between optical and radio polarization, although the optical varies much more with much larger amplitude, which reflects a variability property of blazers. Variability amplitude in blazers is frequency dependent, is larger at larger energies. And it can reach 3C279 is one of the most polarized. You can see it here. It reaches polarization linear percentages of 40%, which is not every blazer is polarized to this level. But very frequently, they do in high polarized states, they often reach this value. On the right, you see a blazer of a little bit different type. This type of blazer is the so-called high frequency peak blazer. 3C279 has a spectrum peaking in the far infrared. And so it's known as a low frequency peak blazer. ES1959 is a high frequency peak blazer. The synchrotron spectrum peaks at x-rays, at soft x-rays, or hard x-rays during outbursts. It's a little bit closer. It's a more famous TV source. In fact, this is a low TV source, exactly because it's closer. It's rather close by. It's been detected at TV energies at many epochs. And here is just the flux on top of two epochs, two monitoring epochs in 2009 and the accompanying polarization, which is seen to correlate in very different ways in the two occasions. It's more or less constant as opposed to flux increase in the first epoch, while in the 2B epoch, there is a good correlation between flux and linear polarization. And this, of course, is related to the different stochastic behavior of jets. There is no predictability of the blazer jets behavior. So it's to be expected that correlations may change with emission state, with epoch, with time. And the position angle is rather constant in this case. This is a very nice example of a correlated radio to x-ray, in fact, to TV energies of monitoring of BLLAC. This is the prototype of the blazer class. It's a nearby object, which makes it an easier source to observe, to detect the TV energies. This is a campaign that started, was meant to be a multilagent campaign together with a radio monitoring of radio knots or the behavior, temporal behavior, of radio components in this source. So on top, you see a map, time-resolved radio maps that follow the central source, which is indicated by the 0 milli-act-seconds. That's the nucleus, the location of the coastal galaxy of this blazer. And then you see the development with time of different radio components. In the bottom, you see the results of a campaign, a multilagent campaign, that follow the behavior of this source during the simultaneous epochs, as you can see in the bottom, more or less the years. You follow the years. They develop between the beginning of 2005 and the end of 2006. And you see the x-ray-like, x-ray spectral index, the optical-like curve, and the radio. And then a blow-up, the two vertical dashed lines, the stripe delimited by those two lines is blown up on the right. So you have a better view of the variability in these bands. There is a little arrow in the x-ray panel, which is a blow-up of the x-ray-like curve, that shows you the detection of photos, TV photos. And in the, so you see, again, a blow-up of the optical in our band, and then the polarization behavior. So you see that you have the arrow in the h-panel indicates the crossing of the, or the radio knot crossing of the core area, which you can map, you can associate with the second map on top. Just before that epoch, the flare has developed. The rotation, the position angle of the polarization has undergone an almost complete cycle, almost complete rotation. At the same time, the polarization percentage flares up and goes to a minimum and flares up again. This is a complex behavior that was explained with a empirical heuristic model that you see in the right. In the right panel indicates, shows up a jet, where there is a helical magnetic field. And the plasma, the departure of the knot from the center is accompanied by this, by the flare at all other wavelengths. So then you see the knot appears, becomes visible in radio, and it triggers a second flare, which you can see here, in X-rays and R-band around epoch 2005, around the end of year 2005. So in this present case, particular case, the availability of polarization information guided the modeling of the behavior of the outburst, of the development, and the start and development of the outburst. This is a very nice observation. Here you see one of the blazers that have the most extreme synchrotron behaviors. As I was saying, blazer synchrotron spectrum have, you see here the typical spectral behavior of a blazer, that component, a first component, which is produced, the first hump that is produced by synchrotron radiation and the second hump that is inverse content of synchrotron photons, as in this case, in Markayla 501, or other external photon fields, as I was saying at the beginning, like a creation disco or emission lines. The present case, this particular blazer has a synchrotron component peaking in the X-ray domain. So that's why it's called extreme. Well, that's not exactly the reason. There are several, many blazers that have soft X-ray peaking synchrotron components. The maximum of the synchrotron component is between the ultraviolets located between ultraviolet and soft X-rays. Well, in some cases, in a few subclass, during outbursts, this synchrotron component flares up, and the synchrotron maximum shifts to up to 100 kV, or perhaps more. That's why they're called extreme synchrotron blazers. So of course, the very high energy synchrotron maximum are, of course, the node betrayed the presence of extremely energetic electrons. And therefore, the Klein-Nichina domain, the Klein-Nichina effect, suppresses the TV, the inverse quantum component very dramatically. So generally, the very high energy of the inverse quantum component in these blazers has a lower luminosity than the synchrotron component, because it is highly Klein-Nichina suppressed, as you can see here. So these are the three spectra taken at different years. So 16 April 97 is when Beppu sucks the satellite. So one of the biggest, the most spectacular flares of this source, and then in subsequent years, the synchrotron component cooled down, and accordingly, the synchrotron maximum shifted back to lower energies. Recently, so this source is known to be a polarized source. You see here the polarization degree in optical. In this view, if you look at the four left panels, you see a historical optical liker in R-band in the top, historical linear polarization degree in the bottom, and then the polarization angle and the x-ray flux in the fourth bottom panel. The optical polarization is significant. It goes up to 6% or more. And recently, this source has been selected for observations with X-PEC. They recently launched the x-ray polarization satellite. And very interestingly, X-PEC detected a level of polarization that is higher than both radio and optical. You see this on the right. You see a polarized spectral energy distribution of this source. It is just the spectrum of the polarized radiation, which is, on average, 2% in radio and optical. The triangles indicate the host subtracted value of the polarization in optical. And compared with the x-ray, the x-ray polarization is much higher. And this led the authors, Lyodakis et al, who, if I'm not mistaken, Lyodakis may have talked about this source and this observation precisely during one of these webinars, if I'm not mistaken. But the level of the polarization and the alignment of polarization angle and jet direction as derived by the radio maps of this source indicate shock-accelerated energy-stratified electron population in this source. We now go to remove from blazers two gamma-reversed, which are also very interesting polarized sources. This is one of the most interesting because it is so distant. I think this is the farthest gamma-reversed for which polarization has been measured. And you see here in the top left, you see a light curve. You see the x-ray and the optical light curves with the typical afterglow. This is not the gamma-reversed. This is the optical and x-ray counterpart, the bright afterglow that is quick decreasing. At some point, it starts decreasing faster than at the beginning, the so-called break point, the temporal break of the afterglow light curve, which is probably, possibly, physically related to the physical extent, to the physical geometry of the jet. Allegedly, the t-break, the time at which the afterglow starts fading more quickly, is related, gives you a measurement, an estimate, of the jet opening angle. It's interesting because, allegedly, some models, this is a model-dependent statement, some models foresee a change of the polarization position angle at that time when that geometrical physical limit is reached. And apparently, this is what is seen in polarization measurements of this object. If you look at the bottom curves, the bottom temporal curves show you in red, the red balls show you the polarization measurements, which have been taken, they are separated by, they are taken at two different epochs. And if you look at the position angle in the bottom, the prediction, the horizontal dashed lines indicate the model predictions. Apparently, the measurements agree very satisfactorily with this prediction of the position angle variation across the temporal break, across the physical size of the jet. And the same, there is a comparison here, the data compared with the measurements of closer gamma-reviews, gamma-reburst, 18 October 2009, that are actually different. The polarization percentage is different, but the behavior of the position angle is consistent. Quite interestingly, I should say maybe curiously, because we don't really know what to make of it. Well, it's a very complex type of measurement. This gamma-reburst, 24 October 2012, showed even some circular polarization at low level. But there is apparently a measurement that can be seen as significant of circular polarization. It's the only case among gamma-rebursts. And it is anyway quite difficult, quite complex to interpret. So it would be nice, in fact, it would be interesting to be able to measure more circular polarization, let alone linear polarization, of gamma-rebursts. And particularly, if we could do that early enough, sufficiently early epochs after the explosion. This has been done, as you can see here, in this example of a TV gamma-reburst. This is one of the very few gamma-rebursts detected so far at TV energies. You see on the top left, you see the collection of the multiravellant likers, from TV from radio to up to, you should be able to see here, yes, there is also the magic points. So it's radio to TV likers. And on the right, you see the spectral energy distributions. The bottom shows in three bands, V, R, and I, the polarimetric measurements taken very early, which started very early on. As you see, 500 seconds after the gamma-reburst, there is a detection in all bands with this telescope. This is a 2-meter telescope, plus the polarimeter that used to be mounted on this telescope. It's been mounted till recently, Ringo 3, which unfortunately could not follow the source later on. Anyway, it's already very interesting to have this early detection. It's just a pity that there is no more. This is at the level of a few percent as seen before, as in the previous cases that we've seen in the previous slide. So apparently gamma-reburst polarization does not reach as high values as in blazers, which tells a lot about the magnetic fields and the role of magnetic fields and geometry in these jets, blazer and gamma-reburst jets, which must be very different in their behavior clearly. This is still an interesting slide. Unfortunately, there is no detection, but this is the first measurement of X-ray polarization. Again, we'll expand, of course, gamma-reburst. Again, it's the brightest of all times. It is again the 9th of October 2022 gamma-reburst. You see the beautiful Leica, the prompt event Leica on the left, that shows both the prompt event and the early part of the afterglow in hard X-rays, between 80 and 320 kV. The bottom shows the swift detection. Swift didn't detect this source immediately. This is the top, it's a con-swinged detection. There was a Fermi-GBM detection, and then Swift followed, but Swift monitored the afterglow for a very long time, as you can see here, up to 10 to 7 seconds, several months after the explosion. On the right, you see the balls are not the detections. Those are estimates of the background, and you see the arrows, the three arrows indicate the three upper limits obtained corresponding with these three different estimates of the background. The bottom plot is just to put the source in context. Just to show, it is just a comparison of the isotropic gamma-ray energy of this source, just to give you an idea of the power of this source, compared with a sizeable sample of long gamma-ray bursts with redshift. This is not all gamma-ray bursts, long gamma-ray bursts with redshift, so that we know of, it's a good fraction of them. And our guy, our 9 October guy, is clearly a very big, a very big, very, very strong source. There was, as you know, virtually all long gamma-ray bursts are associated with supernovae with core collapse, stripped envelope core collapse supernovae, namely supernova explosions of stars that have lost completely their hydrogen and helium envelopes before explosion. So they explode as type 1c supernovae, stripped envelope supernovae. They lack completely hydrogen and helium features in their ejecta. And one of the closest ones and best studied was this one at relatively low redshift. Obviously, in fact, quite low redshift. It's very famous because the early light curves, the X-ray and ultraviolet early light curve of this source indicated a component, a very bright X-ray and UV component, which has debated interpretation. It's not clear if it is a shock breakout of the supernova or non-thermal jetted emission. It is not clear and not agreed upon. Certainly the supernova, the ensuing supernova that you see here in the panel that says UVOT, those are optical and ultraviolet light curves obtained by UVOT of the counterpart, starting one day, 10 to the 5 seconds after the explosion, the light curve is obviously dominated by the supernova. And on the right, you see the measurement of the polarization. There is polarization associated with this supernova at the level of a few percent. In fact, you should particularly look at the bottom panel because that's corrected polarization, corrected for the contribution of host galaxy and afterglow. And it shows a highly variable polarization of the supernova with time. So these are also possible. Very supernova, core collapse, supernova, have potentially very highly asymmetric explosion. They can be very asymmetric sources because the exploding star is rotated, is a massive star with rotation. So rotation is expected to imprint some sort of directionality in the explosion, particularly if the supernova is associated with a gamma-ray burst, where we know for sure that there is a preferential direction, which is the direction of the jet. So you do expect some high level of asymmetry in general in core collapse supernovae, and in particular, in those associated with gamma-ray bursts. So you expect to be able to measure some significant polarization. And this is what's left after a core collapse supernova after many, many years. This is Kasei, one of the most famous supernova remnants in the galaxy. It is particularly important because Kasei is almost certainly the result of a core collapse supernova. And you still see that even the idea, even the trace of some directional structure that has been expelled emitted by this source, there is this jet that you see in optical and radio light, the bottom left image. So the bottom left image also has a composite optical and radio image, and there are overlaid some small white lines that indicate the intensity of the polarization. There are many polarimetric maps done with this. These are actually, this is actually millimetric polarization. This is sub-millimetric polarizations taken at SCUBA. And they indicate a very interesting, very nice pattern that has been interpreted as a pracer of dust formation and evolution. The same source in X-rays you see is shown on top. That's the chandra image in purple is the chandra total light image of Kasei. Sorry, in blue, you see the blue features is the X-ray total light. The purple one, which you see also as an intensity map on the right, is the polarization. Is the X-ray polarized image of Kasei as measured by X-pad. That shows some very irregular, non-uniform pattern that give you an idea. They are interpreted as being related to the shock compression and magnetic field compression along the rim of the supernova line, a very complex pattern, very interesting. And the polarization is also quite high. It's between 2% and 5% in X-rays and up to 30% in radio, in sub-millimetric. Sorry, you will have five more minutes. OK, so I will show now the VST pole, which is the next polarimetric facility at Iso. It's a camera, it's a large field of view camera for the VLT survey telescope. This is the, you have various images here of the Paranal Summit with the four VLT units that you see here in various perspectives. And you see also the VST telescope, the VLT survey, 2.6 VLT survey telescope in these same images, which has recently changed hands. It is now managed by ENAV and no longer by Iso. So we are building, so far VST has been around for a long time. It has done a lot of survey work with omega-cam, which is an optical total light camera with various filters. The idea is to equip now to fit the VST with a polarimetric large field camera, and possibly to install also the rapid response mode to make the telescope semi-robotic, not semi-robotic. This is improper, to allow the telescope to respond to alerts, to very fast alerts in a very rapid time, just like the VLT, so in a time, in a matter of minutes. So this new camera that we are planning is part, is funded by the recovery, post-COVID recovery plan, and is coordinated, the design and implementation is led by ENAV, by the observatories of Breira and Naples, in particular. As you can see here from the central graph, the transmission of the camera peaks in the V2R band, and so the polarimetry will be maximum, that the maximum sensitivity for polarimetry will be in that band. The camera has a good transmission, also in high band, but then the polarimetric capability, the polarimetric performance, is decreasing quite rapidly in that band. So the purpose of VST-POL will be to enhance the polarimetric capabilities at ESO, and to coordinate with multi-wavelength campaigns of transients, in particular, CTA, in particular TV transients, which will be the prime targets of CTA, and possibly, since it is a large camera, it can be coordinated, it can work together with other large field instruments, like Rubin or Radio Arrays in the future, like a male cat, SKA. I have already illustrated a number of scientific cases where having a sensitive polarimetric facility may make a big difference. I would like to add the multi-messenger aspect of this activity. As you know, later in the spring, the LIGO-Virgo interferometers will resume operations. Occasionally, the air box of the gravitational sources will be some pens of degrees, which can be covered by a camera like VST-POL, a polarimetric camera like VST-POL can be covered in a rather quick time, and in search for polarization of gravitational emitters, and in particular, of these kilonomes, these binary aftermaps of binary neutron star mergers. The only one that we know so far has a low polarization, 0.5%. However, the polarization of these neutron star mergers can be very angle, very highly angle dependent. So we may measure variable, possibly higher polarization, maybe variable with time, depending if we were located in a more favorable viewing angle. So these are my conclusions that I will leave here since I think my time is over, and I will take questions. Thank you very much for your attention.